In a fuel cell, hydrogen gas or other fuel gas and an oxidizing gas containing oxygen are separated by an electrolyte disposed there-between and reacted with each other electrochemically through the electrolyte. Electric energy is extracted directly from electrodes provided on both sides of the electrolyte. In particular, solid polymer fuel cells, which use a solid polymer electrolyte, are attracting attention as a power source for electric vehicles because of their low operating temperature and ease of handling. More specifically, a fuel cell vehicle is installed with a hydrogen storage device (such as a high pressure hydrogen tank, liquid hydrogen tank, or a hydrogen absorbing alloy), and hydrogen supplied from the hydrogen storage device and air containing oxygen are delivered into the fuel cell and reacted together. Electric energy extracted from the fuel cell is used to drive a motor connected to the drive wheels. Since the only exhaust substance is water, a fuel cell vehicle is extremely clean.
In a fuel cell system, energy held by the fuel that cannot be extracted as electric power is converted to heat. If the temperature of the fuel cell rises above a permissible temperature due to this heat, the fuel cell will be degraded. Consequently, fuel cell systems are provided with a cooling system that uses a coolant to absorb heat generated during electric power generation and release the heat outside the system through a radiator or other heat exchanger.
In the case of solid polymer fuel cells, the upper limit of the operating temperature is determined chiefly by the maximum tolerable temperature of the solid polymer electrolyte film and it is necessary to cool the fuel cell stack such that the upper limit temperature is not exceeded.
Japanese Laid-Open Patent Publication No. 05-074477 (page 3, FIG. 1) discloses a technology that limits the amount of electric power extracted from a fuel cell and curbs the amount of heat generated by the fuel cell when the cooling capacity is insufficient relative to the amount of generated heat.
The technology disclosed in said publication is configured to limit the output of a fuel cell plant in accordance with the outside air temperature when the outside air temperature is high and the cooling device cannot cool the fuel cells sufficiently, thereby preventing the fuel cells from being degraded by excessive temperatures.